Large centrifugal forces act on a tire during running at high speed. Consequently, the tire inflates radially outward, causing outer diameter growth. As a result, the edges of a belt ply which receive stress concentrations are subject to occurrence of separation. Therefore, a belt-reinforcing structure having an organic fiber cord(s) spirally wound in the circumferential direction of the tire is conventionally provided radially outwardly of the belt ply as a cover member, whereby the outer diameter growth of the tire during running at high speed is restrained to thereby suppress occurrence of edge separation of the belt ply, obtaining high-speed durability.
As one example of the above conventional belt-reinforcing structure, there is known a belt-reinforcing structure having a center reinforcement layer disposed radially outwardly of a center region of the belt ply and end reinforcement layers disposed radially outwardly of opposite end regions of the belt ply. The center reinforcement layer has a cord of organic fiber greater in elongation (less in elastic modulus) such as nylon, and the end reinforcement layers have an organic fiber cord for which a hybrid cord is used, the hybrid cord having a filament of organic fiber greater in elongation such as nylon and a filament of organic fiber less in elongation (greater in elastic modulus) such as aramid fiber, the filaments being twisted together (see a patent document 1, for example).
By configuring the belt-reinforcing structure as described above, there are advantages that the organic fiber cords of the belt-reinforcing structure can follow lifting at the time of tire vulcanization, and that outer diameter growth in the ends of the belt ply where edge separation occurs is effectively restrained, enabling high high-speed durability to be provided. Further, vibration in the sidewall portions of the tire can also be suppressed, which contributes to a reduction in tire noise.
In recent years, there are strong requirements for further improvements in tire properties with improvements in vehicle performance. Further improvements in the above-mentioned high-speed durability and noise characteristics are also required strongly.    Patent Document 1: Japanese Patent Application Kokai Publication HEI 5-338409